math
posted by ja'Sae .
A shop sells onepound bags of peanuts for $2 and threepound bags of peanuts for $5.
If 9 bags are purchased for a total cost of $36, how many threepound bags were
purchased?

O + T = 9
T = 9  O
2O + 5T = 36
Substitute 9O for T in last equation and solve for O. Insert that value into the first equation and solve for T. Check by inserting both values into the second equation.